The paper discusses our recent experiments focused on the chromium influence on the microstructural changes of iron based alloys under radiation treatment. Our experimental method ¿ the positron annihilation lifetime spectroscopy (PALS) enables an observation of size and density changes of the vacancy type defects in the material microstructure. These defects have been created by implantation of charged particles (He2+). The cascade collisions in the crystal lattice and following Frenkel pair creation have been considered as possible approximation of the neutron flux damage up to 100 dpa in the region up to 1 micrometer from the surface.